Electrostatic coating apparatus



April 7, 1964 w. D. GAUTHIER 3,128,045

ELECTROSTATIC COATING APPARATUS Filed May 31, 1961 2 Shee's-Sheeil 1 April 7, 1954 w. D. GAUTHIER 3,128,045

ELEcTRosTATIc coATING APPARATUS Filed May 31, 1961 2 SheetS-Sheei 2 INVENTOR.

WILLIAM D. GAUTHIER BY #WZM Aoeys United States Patent Oiiliee 3,128,045 Patented Apr. 7, 1964 3,123,645 ELECTRSTATC CATWG APPARATUS William D. Gauthier, indianapolis, llnd., assignor to Ranshurg Electro-@eating Qorp., indianapolis, End., a corporation of indiana Filed May 3l., 1961, Ser. No. 113,928 i9 tlaims. (Cl. Z39-15) This invention relates to apparatus for forming a spray of electrically charged particles and more particularly, it is concerned with improvements in devices for forming a spray of charged particles useful in coating operations. When coating material is fed to such a sprayer, the coating material moves over a surface of the sprayer under the action of appropriate forces to points on said sprayer from which it is discharged in the form of a spray of iinely divided electrically charged particles, the particles being electrically charged as a result of a voltage upon the sprayer itself.

One example of a sprayer which has heretofore been employed comprises an electrically charged rotating metal disk or bell. Coating material supplied to one surface of such sprayer is moved to its periphery by rotating it. From this periphery the coating material is dispersed in the form of a spray of nely divided, charged particles. The voltage applied to such a device is generally essentially independent of spacing between the device and grounded objects and such a device generally has high effective capacity. A disadvantage of such a sprayer is that if a person accidentally approaches it too closely he will receive a highly objectionable shock and a spark discharged from it may ignite inflammable coating material or cause an explosion.

Another example of a sprayer which has heretofore been employed comprises a rotating bell of insulating material having a high resistance electro-conductive coating extending over one of its surfaces. When the bell is rotated, the coating material fed to the bell will move by centrifugal force over a surface of the bell to the discharge edge of the bell. With a charging voltage applied to the conductive coating, the coating material will be dispersed from the discharge edge as a charged spray for electrostatic deposition on the articles to be coated. While such a bell produces a high quality spray of nely divided particles for use in coating operations, if the bell has too large a diameter, either the coating on the bell has to have too high a surface conductivity for safety or too low a surface conductivity for applying a satisfactory charge to the material to be sprayed. Also, the high resistance electro-conductive coating on the bell is subject to destructive scrubbing action when said conductive coating is disposed over the bell surface over which the coating material moves, and accidental impact at the discharge edge may damage the conductive coating and thus adversely affect charging. Further, the relatively high temperature required to process the conductive coating may physically distort the bell.

It is thus the general object of the invention to provide an improved sprayer which will overcome the disadvantages and difficulties just discussed. More specically, it is an object of the invention to provide an improved sprayer which will have sufliciently low effective capacity to minimize shock and fire hazards and yet be suiciently conductive to permit the desired electrical potential on the coating material when it is at the dispersing or charging zone, and which will be highly resistant to the chemical and abrasive action of coating materials. Another object of the invention is to provide charging apparatus having a discharge edge not readily affected by accidental impact. A further object of the invention is to provide a charging apparatus in which the effect of distortion of some of its parts resulting from their manufacture is minimized. Still another object of the invention is to provide a large, safe sprayer from which coating material is discharged with improved uniformity.

The effective capacity of an electrode system is not a measure of that systems ability to store electrical energy (conventionally dened as the electrical capacitance of the system) but rather it is a measure of the propensity or ability of a destructive discharge obtainable from the system to shock personnel, ignite inflammable vapors, or cause an explosion. It is a direct function of electrical capacitance and an inverse function of the distributed electrical resistivity and the sharpness of the electrode, but this concept has no conventional units of measure. lf an electrode is made of metal, its entire capacitance can be essentially instantaneously discharged, and therefore, for safety its size must be relatively small. On the other hand, if the electrode is made of highly resistive material, as an insulating material with a highly resistive or semi-conductive coating, the entire capacitance of the electrode is not instantaneously discharged and some of the stored energy is dissipated by the resistance of the electrode. Therefore, such an electrode may be larger than could otherwise be tolerated. Also, sharpness of exposed portions of the electrode promotes ionization and increases leakage current therefrom resulting in reduced electrode voltage and reduced intensity of discharge.

According to one form of the invention there is provided a spray charging means formed of an electrically insulating material having an electro-conductive, preferably semi-conductive, element carried within it and an outwardly presented coating-delivery face terminating in a discharge portion. The electro-conductive element is adapted for operative connection to a high voltage source and is disposed in a position in which the coating material is brought into operative association therewith preferably adjacent said discharge portion. Means are provided for supplying coating material from a source of supply onto said coating-delivery face and moving it into operative association with said electro-conductive element and to said discharge portion from where said coating material is discharged in the form of a spray of finely divided, charged particles for deposition on the grounded articles to be coated.

Other objects and features of the invention will become apparent from the more detailed description which follows and from the accompanying drawings, in which:

FIG. 1 is a vertical section of an electrostatic charging apparatus having a sprayer embodying the invention, but showing the coating material and electrical power supplies in schematic form;

FIG. 2 is a plan view of the sprayer shown in FIG. 1; and

FIG. 3 is a plan view of a different embodiment of the electro-conductive element shown in FIG. 1.

A typical apparatus embodying the invention is shown in the accompanying drawings, and comprises a sprayer formed from an interconnected annulus lil, rim 12, and disk 14 operatively -connected to a rotatable driving assembly. As shown in FIG. `l, the annulus `10 comprises a pair of annular rings 15 and 16 formed from electrically insulating material, such as epoxy bonded liber glass or the like, which is resistant to abrasive action and attack by the solvents and other ingredients which may be present in coa-ting materials to be sprayed. The inner circumference of the ring :16 is spaced radially outwardly from the inner circumference of the ring 15, aud the outer circumference of the ring 16 extends around the outer circumference of the ring 1S and projects axially therebeyond to provide a step dam or rib 18 extending around the outer circumference of the annulus 10.

As shown, the dam 18 has an annular face parallel to and axially spaced from the upwardly presented face of the ring 15. One function of the dam 18 is to overcome any lack of uni-formity in the distribution of coating material that arrives at the inner portion 18a of the dam. For example, it aids in eliminating any gaps in the coating material film arriving at 4the foot of the inner dam portion 18a and feeds the coating material uniformly over the top of the dam to the outer sharp edged periphery 1817, which is at `or adjacent the sprayforming zone. Another advantage of the dam 18 is that when it is of the proper contour, field concentration is limited to only the spray-forming zone. This peripheral edge may be formed as a -one-quarter round edge as shown, or it may be in the form of an undercut knife edge. The use of an insulating surface on the outwardly disposed edge `of the sprayer tends to increase the electrical gradient at the spray-forming zone and prevents damage to the terminus of 4the electro-conductive element adjacent thereto. An advantage of the one-quarter round edge is that besides providing an increased field concentration lat the spray-forming zone, it also provides a sprayer edge having a shape which is not easily damaged.

Interposed between the rings and 16 is an electroconductive element y having a distributed high resistance, which may be conveniently formed as a semi-conductive, resinous coating applied to the ring 1'5. Such a coating may comprise finely divided, electrically conductive, solid particles, such as finely divided carbon, carried in a resinous binding material, such as a thermosetting resin. As Will be appreciated, the element 20 may thus be compounded to provide the said element with a predetermined resistance by controlling .the amount of carbon added to the resinous binder in which it is carried. Desirably, the element 20 provides a member having a multi-megohms per square resistivity, but the resistivity should not be so great as to prevent maintenance of an adequate charging potential at the sprayforming zone. As shown in FIG. 1, the element 20 is interposed between and bonded to the adjacent faces of the rings :15 and 16. In the event element 20 is applied to ring 15 as a resinous coating the ring 16 should be cemented to .the resinous coating with a material such as nonconducting epoxy cement. The major portion of the element 20 is thus sandwiched between the rings 15 and 16 with its inner edge portion disposed radially inwardly from the inner circumference of the ring 16 in an unshielded position. Desirably, the outer edge of the element 20 terminates in the plane of the coating-delivery face of the ring 15 between the outer circumference of said ring and the step dam 18.

The distributed resistivity of the element 20, coupled with the fact that said element is sandwiched between the pair of electrically insulating rings 15 and 16, results in the sprayer having a low effective capacity. Thus the sprayer may have a relatively large diameter, yet possess suicient conductivity to prevent an excessive voltage drop across the element so that an eiiicient Vand effective iield concentration can be maintained in the spray-forming zone without creating a safety hazard. Sandwiching these members tends to overcome warpage o-f the individual members which may have resulted from previous manufacturing operations and provides a rigid structure.

The inner edge of the ring 15 is connected to the rim 12 as by a nonconductive cement, such as epoxy cement, with the upper edge of said rim being shouldered as at 22 to dispose its upwardly presented edge coplanar with the coating-delivery face of the ring 15. For reasons that will become more apparent hereinafter, the rim -12 is provided with an annular trough 24, the outer side wall of which slopes upwardly at 26' and terminates in an annular shoulder 28. A sloping wall extends from the shoulder 28 to the upwardly presented face of the rim 12.

'The rim 12 is connected as by pins 32, formed of '4 electrically insulating material, to the disk 14. To facilitate assembly of the apparatus, the element 20 is bonded to the :ring 15, and the ring 16- is then bonded to the element 20. The rim 12 is pinned to the disk 1'4, and the ring 15 with the element 2G* and ring 16 connected thereto is then bonded to 4said rim. In this manner, the ring 16, which is generally coplanar with the pins 32, does not interfere with the pinning of the rim and disk.

As shomi, the upwardly presented face of the disk 14 is generally coplanarl with the coating-delivery face of the ring 15, and the outer circumference of said disk overlies the rim shoulder 28 in axially spaced relation thereto. Thus, upon rotation of the sprayer, coating material, for exam-ple, a paint such as is used in commercial spray painting operations, fed to the trough 24 by a nozzle 34 from a source of supply 35 will, under the action of centrifugal force, climb the rim walls 26 and 30 onto the coating-delivery face of the ring 15. In most cases coating materials are not extremely electrically conducting, and where such materials are used, the supply 35 and possibly some intermediate point of `the supplying line extending from supply 35 should be electrically grounded. However, in the event that coating materials are of a type containing relatively high percentages of aluminum pigment or by virtue of any other ingredient having good electrically conducting properties, the supply 35 and .the supply line should be electrically insulated from ground. In this latter case there is some sacrice in operational safety of the equipment but it is still safer than, for example, conventionally used electrostatic disk-type coating apparatus. The Weir formed by the disk 14 overlying the rim shoulder 28 will confine the movement of said coating material to the walls of said rim and prevent said coating material from being thrown generally axially outwardly from the upwardly presented edge of the rim vvall 26. The centrifugal force created by the rotation of the sprayer causes the coating material to move across the coating-delivery face of the ring 15 to the edge thereof, with said coating material passing in operative association with the outer peripheral edge of the conductive element 2t). A high voltage is impressed on the element 20 by a brush connection 37 leading from a high lvoltage source 29. Thus, the coating material rnoving to the edge of the ring 15 will come into operative association with the element 2t), and will move over the dam 18, which is formed of electrically insulating material and is disposed radially outwardly of the element Ztl', and to the edge 1gb from which it is dispersed as a spray of finely divided, electrically charged particles yfor deposition on the grounded articles to be coated. Desirably, a high voltage of at least 40,000 volts, and preferably 70,000 volts or more, is supplied from the voltage source 29 (generally from the negative terminal thereof) through resistance, preferably of lthe order of several megohms per kv. applied with a substantial portion of said resistance adjacent brush 37, and through the blush 37 to the sprayer for establishing the required electrical gradient between the sprayer and the articles to be coated. -If this equipment is used with a 'voltage supply having an output voltage which decreases appreciably as current increases, series resistance may be reduced.

To effect rotation of the sprayer, the disk 14 is interconnected to a drive shaft 39 by an assembly all of whose components at the sprayer end should preferably be formed from electrically insulating materials. Such assembly comprises a hub iti connected to the disk 1d by a plurality of screws 42. The hub projects axial-ly through the rim 12 and has a shouldered opening 44 formed in its upper end for the reception of a screw 45 connecting said hub to a drive coupling 47 mounted on the rotatably driven drive shaft 39 by nou-conductive cement '49. As shown, the driving torque of the coupling 47 is transmitted to the hub 40 by a key 48 received in mating keyways formed in Vsaid hub and coupling.

The coupling 47 is journaled within a bearing 52 which is cemented to a sleeve 58. A platform 56 is secured to the bearing `52 by screws 54. The platform serves to support the coating material iine leading to the nozzle 34 and the high voltage lead-in for the brush 317.

iin certain applications it may be desirable to drive the sprayer by an air motor (not shown) which is enclosed in housing 57. The drive shaft 39 and motor may be housed in the sleeve 58 so -that the exhaust air will move within the sleeve 58y to pass through openings 60 formed in the bearing 52 thence outwardly through the spacing 63 between the hub 40 and bearing 52.

The sprayer is illustrated and has been described herein in an upright horizontal position. However, it may be oriented in any plane, and it may be oriented in an inverted position with the coating-delivery face presented downwardly. To this end, the drive coupling 47 is provided with an annular rib 65 a-t its end remote from the hub 40. Thus, should the sprayer be oriented with the coating-delivery face of the ring presented downwardly, the sprayer will be connected to the drive shaft 39' by the cement 49 interconnecting the coupling 47 to said drive shaft, but in the event of a break in such cement, or in the shaft 39, or in the shaft T39-motor interconnection, the sprayer would still be prevented lfrom falling off the drive shaft by the rib 65 on the coupling 47 engaging the bearing 52.

An electro-conductive member constituting a modiiied Iform of the member 20 is illustrated in FIG. 3, and may be substituted for said member 20. The member Z0 is adapted to be formed on the ring 15 and comprises a pair of concentric rings 67 and 69 of a semiconductive coating applied to the ring 15. The rings 67 and `69 are disposed on the ring i5 adjacent its inner and outer edges whereby the ring 67 may make contact with the brush 37 and the ring 69 will be disposed adjacent the inner portion 18a of the dam. The rings 67 and 69 are electrically interconnected by electro-conductive members, `conveniently in the form of small diameter wires 70 recessed in the ring 15 and electrically connected to the rings 67 and 69.

While the sprayer illustrated has a flat annular configuration, it is to be understood, of course, that said sprayer may be contoured in a manner to give non-annular, generally frustroconical, or generally tubular coniigurations.

While the specic embodiment of the invention has been described as a rotating type of sprayer, the principles and concepts of the invention may also be employed in a non-rotating type of sprayer, such as is disclosed in Miller U.S. Patent No. 2,748,0l8, and may be used with sprayers utilizing both liquid and solid particled materials.

What is claimed is:

l. -ln a charging apparatus for coating material, a spray-(forming and charging device substantially the entire exterior of which is formed of insulating material having an outwardly presented surface terminating in a discharge edge, said device having an extended electroconductive element spaced lf-rom said surface and in large part surrounded with the insulating material, means for connecting said element to a high-voltage source, and means for supplying coating material to said outwardly presented surface at a point thereon spaced from said discharge edge and for causing such material to move over and in contact with said surface through chargerecei-ving association with said element to said discharge edge from where it is discharged as a charged spray having the same polarity as the electro-conductive element.

2. ln a charging apparatus for coating material, a pair of insulating members, one of said insulating members terminating in a discharge edge, an electro-conductive element interposed between said members, said element being adapted to be connected -to a high voltage source, means for supplying coating material from a source of supply onto the outwardly pre-sented face of one of said members, and means for moving the coating material over and .in contact with said face of one of said members into operative association with said element and to said discharge edge where the coating material is discharged as a charged spray having the same polarity as the electrocond-uctive element.

3. In a charging apparatus for coating material, a spray charging means of electrically insulating material having an electro-conductive element carried within it and an outwardly presented surface terminating in a rib with a discharge edge, said element being adapted for operative connections to a high voltage source, and means for supplying coating material from a source of supply onto said outwardly presented surface and moving it into operative association with said element and over said rib to said discharge edge from where it is discharged as a charged spray.

4. The invention set forth in claim 3 wherein said charging means has a rounded surface connecting said discharge edge and the face of said spray charging means opposite said outwardly presented surface, and said rounded surface is entirely inwardly of said discharge edge.

5. In a charging apparatus for coating material, a spray charging means of electrically insulating material having electro-conductive means carried within it and an outwardly presented surface terminating in a rib with a discharge edge, said charging means being adapted for operative connection to a high voltage source, and means for supplying coating material from a source of supply onto said outwardly presented surface and moving it into operative association with said electro-conductive means and over said rib to said discharge edge from where it is discharged as a charged spray.

6. In a charging apparatus for coating material, a pair of generally annular electrically insulating members, an electro-conductive element interposed between said members, said element being adapted to be connected to a high voltage source, means for supplying coating material from a source of supply onto an outwardly disposed face of one of said members, and means for moving said coating material over and in contact with said outwardly disposed face into operative association with said element and toward the periphery of said members where the coating material is discharged as a charged spray having the same polarity as the electro-conductive element.

7. The invention as set forth in claim 6 in which said electro-conductive element comprises a pair of generally concentric semi-conductive rings disposed adjacent the inner and outer circumferences of said pair of insulating members and electrically interconnected by one or more electro-conductive members.

8. The invention as set forth in claim 6 in which said electro-conductive element comprises a thin annular layer of semi-conductive material.

9. In a charging apparatus for coating material, a member having outwardly presented rst and second electrically insulating surfaces, an electro-conductive element carried within said member between said surfaces and adapted to be connected to a high voltage source, means for supplying liquid coating material from a source of supply to said first insulating surface, and means for connecting said member to a rotational driver for rotating said member to cause said coating material to move over and in contact with said rst surface into operative association with said element and toward the periphery of said member where the coating material is discharged as a charged spray having the same polarity as the electroconductive element.

10. In a charging apparatus for coating material, first and second annular members formed of electrically insulating material, said second member having its inner edge terminating outwardly from the inner edge of said rst member and having a circumferentially extending rib at its outer edge projecting axially beyond the plane of said first member, an electro-conductive element interposed between said members, the inner edge of said element terminating intermediate the inner edges of said first and second members for connection to a high voltage source and the outer edge of said element terminating adjacent said rib adjacent the plane of the outwardly presented face of said first member, means for supplying coating material to said outwardly presented face of said first member from a source of supply, and means for connecting said members to a rotational driver for rotating said members to cause said coating material to move over the face of said first member into operative association with said element and toward said rib where the coating material is discharged as a charged spray.

11. The invention as set forth in claim wherein the outwardly presented edge of said rib is a discharge edge, said rib has a rounded surface interconnecting said discharge edge and the face of said second member opposite said discharge edge, and said rounded surface is entirely inwardly of said discharge edge.

12. In a charging apparatus for liquid coating material, first and second annular members formed of electrically insulating material, said first member having a coatingdelivery face and a second face interconnected to said second member, an electro-conductive element interposed between said members, the outer edge of said second member terminating in a rib circumscribing the outer edge of said first member and projecting axially beyond the coating-delivery face of said first member, the inner edge of said element being adapted for connection to a high voltage source, an annular rim secured to said first member adjacent its inner edge and defining a fiuid-iiow passage extending between a supply of liquid coating material and said coating-delivery face, and means for connecting said members to a rotational driver for rotating said members to cause said coating material to flow over said coating-delivery face into operative association with said element and toward said rib where the coating material is discharged as a charged spray.

13. The invention as set forth in claim 12 in which said means comprises a hub connected to said rim and extending through the plane thereof for connection to a rotational driver.

f14. In a charging apparatus for coating material, a spray charging means of electrically insulating material having electro-conductive means carried within it and an outwardly presented surface terminating in a discharge edge; said charging means being adapted for operative connection to a high voltage source; a rim of insulating material connected to said spray charging means for supplying coating material thereto from a source of supply; and means for interconnecting said charging means to a rotational driver for rotating said charging means to cause said coating material to move toward said discharge edge where it is discharged as a charged spray, said means comprising a hub rigidly interconnected to said rim and extending axially thereof, a driver coupling rigidly connected to said hub and fixedly mounted on a rotatable drive shaft, and a bearing rotatably supporting said coupling, said hub, coupling, and bearing and their interconnecting means being formed from electrically insulating material.

"15. The invention as set forth in claim 14 in which said hub is interconnected to said rim by a disk connected to said rim by connecting means normal to the axis of rotation of said charging means and connected to said hub by connecting means parallel to said axis of rotation.

16. The invention as set forth in claim 14 in which the ends of said coupling project beyond the extent of said bearing with one of said ends being connected to said hub and the other of said ends having means engageable with the face of said bearing remote from the hub for axially retaining said coupling in said bearing.

17. In a charging apparatus for coating material, a first insulating member having an outwardly presented surface, anv electro-conductive element connected to a second surface of said first insulating member, said element being adapted to be connected to a high voltage source, a second insulatinfT member extending over at least a portion of said element and connected thereto, the termination of one of said insulating members defining a discharge edge, means for supplying coating material from a source of supply to said outwardly presented surface, and means for moving the coating material over and in contact with said outwardly presented surface into operative association with said element and to said discharge edge where the coating material is discharged as a charged spray having the same polarity as the electro-conductive element.

18. In a charging apparatus for coating material, rst and second annular disc-like members formed of electrically insulating material, said second member having its inner edge terminating outwardly from the inner edge of said first member and having an outer discharge portion beyond the outer edge of said first member, an electro-conductive element interposed between said members, the inner end of said element terminating intermediate the inner edges of said first and second members and there being connected to a high voltage source and the outer end of said element terminating adjacent the plane of the outwardly presented face of said first member and adjacent said discharge portion, means for supplying coating material to said outwardly presented face of said first member from a source of supply, and means for rotating said members to cause said coating material to move over `and in contact with the outwardly presented face of said first member into operative association with said element and towards said discharge portion where the coating material emanates as a charged spray having the same polarity as the electro-conductive element.

19. In a charging apparatus for liquid coating material, first and second annular members formed of electrically insulating material, said first member having a coating delivery face, an electro-conductive element interposed between said members, the outer edge of said second member terminating in a discharge portion circumscribing the outer edge of said first member, the inner end of said element being adapted for connection to a high voltage source and the outer end of said element terminating adjacent said discharge portion, an annular rim secured to said first member adjacent its inner edge and defining a fluid-dow passage extending between a supply of liquid coating material and said coating-delivery face, and means for connecting said first member to a driver for rotating said first member to cause said coating material to flow over said coating-delivery face into operative association with said outer element end and toward said discharge portion where the coating material emanates as a charged spray.

References Cited in the file of this patent UNITED STATES PATENTS 

2. IN A CHARGING APPARATUS FOR COATING MATERIAL, A PAIR OF INSULATING MEMBERS, ONE OF SAID INSULATING MEMEBERS TERMINATING IN A DISCHARGE EDGE, AN ELECTRO-CONDUCTIVE ELEMENT INTERPOSED BETWEEN SAID MEMBERS, SAID ELEMENT BEING ADAPTED TO BE CONNECTED TO A HIGH VOLTAGE SOURCE, MEANS FOR SUPPLYING COATING MATERIAL FROM A SOURCE OF SUPPLY ONTO THE OUTWARDLY PRESENTED FACE OF ONE OF SAID MEMBERS, AND MEANS FOR MOVING THE COATING MATERIAL OVER AND IN CONTACT WITH SAID FACE OF ONE OF SAID MEMEBERS INTO OPERATIVE ASSOCIATION WITH SAID ELEMENT AND TO SAID DISCHARGE EDGE WHERE THE COATING AMTERIAL IS DISCHARGED AS A CHARGED SPRAY HAVING THE SAME POLARITY AS THE ELECTROCONDUCTIVE ELEMENT. 